A radio subsystem of a mobile radio system is generally comprised of one or more radio signal transceivers TRX, an antenna part which is common to several transceivers, and a control part. The transceivers include the radio technical units, such as modulator/demodulator, frequency synthesizer, and amplifier units, while the control part includes those controlling and switching devices required to operate the transceivers and to divert calls to/from the mobile services switching centre (MSC) of the system.
Those digital TDMA-systems developed in recent times in Europe and North America also include units for channel coding and decoding in conjunction with transmission and reception, time-slot allocation (e.g., burst generators) and recovery of data from a given time-slot (e.g., symbol detectors, and possibly also equalizers) and speech coding and speech decoding units. In these digital systems, the aforesaid units are controlled by the control unit when transmitting and receiving signals on the basis of then allocated channels. Consider, for instance, that the frequency synthesizer must be tuned to the correct receiving frequency. This means that both frequency and time slot allocation must be controlled in the transceiver units. Furthermore, it is necessary to be able to control the burst generator and the symbol detector in the receiver so as to enable the signal burst intended for a given channel to be recovered from the time-slot determined.
The base station may contain several such transceiver units and one such given unit need not be allocated a given radio channel or other radio parameter (time-slot, frequency, power output, etc.), but can be controlled generally by the control unit to transmit/receive on all of the radio channels allocated to the base station.
Each base station of a cellular system may be allocated a particular set of radio channels and a given radio coverage in a given cell or several sector cells. It is generally known to use a single antenna unit for a plurality of transceiver units within a base station. The individual transmitter units may be connected to the common antenna via coaxial connections and a so-called filter combiner or hybrid combiner.
In other cases, a high linear amplifier intended for a plurality of carrier frequencies is used prior to delivering signals to the antenna via a coaxial connection.
Although this enables the antenna construction of the base station to be simplified, it also results in unnecessary power consumption, since it is necessary to compensate for attenuation of the radio signal caused by the coaxial connection and the combiner with greater power in the output amplifier of the transmitter part. In the other case, which uses a high linear amplifier without a combiner, significant power losses are experienced not only in the coaxial connection but also in the amplifier. Furthermore, the use of a filter combiner, which has lower losses than a hybrid combiner, has, instead, the disadvantage of not permitting any desired frequency combination to be used in the transmitter, because the filters must be designed to be selective for a given transmitter frequency.
The inclusion of transceiver modules in a radio base station has earlier been proposed; see for instance U.S. Pat. No. 4,694,484. Each such module is assigned unique parameters with regard to its function in the radio system, depending on the location of these transceivers and the antenna configuration used.
The radio system described in the aforesaid patent is an FDMA-system in which a channel corresponds to a specific radio frequency. The system also includes a fixed antenna unit comprised of sector antennas each illuminating a cell, and associated combiners (cavities 610-614, FIG. 6). A number of such modules in a base station can be controlled by the base station controller so as to transmit/receive over certain radio channels and in accordance with those requirements demanded by the antenna unit in the base station. The module can be controlled so as to be operative for both transmission and reception of radio calls, which is the most usual mode of operation, or solely to receive and page radio signals. This known structure is able to reduce greatly the number of complicated connections between the different base station units, although the disadvantage of needing to adapt the individual modules to the demands of the antenna unit still remains.